The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to a local coil which may be used to provide localized application of an RF excitation pulse and the localized reception of the NMR signals produced in an NMR scanner system.
Any nucleus which possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency (Larmor frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the magnetogyric constant q of the nucleus). Nuclei which exhibit this phenomena are referred to herein as "spins".
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.z), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment M.sub.z is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a magnetic field (RF excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.1, which is rotating, or spinning, in the x-y plane at the Larmor frequency. The degree to which the net magnetic moment M.sub.z is tipped and, hence, the magnitude of the net transverse magnetic moment M.sub.1 depends primarily on the length of time and magnitude of the applied RF excitation field B.sub.1.
The practical value of this phenomenon resides in the signal which is emitted by the excited spins after the RF excitation signal B.sub.1 is terminated. In simple systems the excited spins induce an oscillating sine wave signal in the receiving coil. The frequency of this signal is the Larmor frequency, and its initial amplitude, A.sub.0, is determined by the magnitude of the transverse magnetic moment M.sub.1. The amplitude, A, of the emission signal decays in an exponential fashion with time, t: EQU A=A.sub.o e.sup.t/T* 2
The decay constant 1/T*.sub.2 depends on the homogeneity of the magnetic field and on T.sub.2, which is referred to as the "spin-spin relaxation" constant, or the "transverse relaxation" constant. The T.sub.2 constant is inversely proportional to the exponential rate at which the aligned precession of the spins would dephase after removal of the RF excitation signal B.sub.1 in a perfectly homogeneous field.
Another important factor which contributes to the amplitude A of the NMR signal is referred to as the spin-lattice relaxation process which is characterized by the time constant T.sub.1. This is also called the longitudinal relaxation process as it describes the recovery of the net magnetic moment M to its equilibrium value along the axis of magnetic polarization (z). The T.sub.1 time constant is longer than T.sub.2, much longer in most substances of medical interest.
The NMR measurements of particular relevance to the present invention are called "pulse NMR measurements". Such NMR measurements are divided into a period of excitation and a period of signal emission. Such measurements are performed in a cyclic manner in which the NMR measurement is repeated many times to accumulate different data during each cycle or to make the same measurement at different locations in the subject. A wide variety of preparative excitation techniques are known which involve the application of one or more RF excitation pulses (B.sub.1) of varying magnitude and duration. Such RF excitation pulses may have a narrow frequency spectrum (selective excitation pulse), or they may have a broad frequency spectrum (nonselective excitation pulse) which produces transverse magnetization M.sub.1 over a range of resonant frequencies. The prior art is replete with excitation techniques that are designed to take advantage of particular NMR phenomena and which overcome particular problems in the NMR measurement process.
When utilizing NMR to produce images, a technique is employed to obtain NMR signals from specific locations in the subject. Typically, the region which is to be imaged (region of interest) is subjected to a sequence of NMR measurement cycles which vary according to the particular localization method being used. The received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. To perform such a scan, it is of course, necessary to elicit NMR signals from specific locations in the subject. This is accomplished by employing magnetic fields (G.sub.x, G.sub.y, and G.sub.z) which have the same direction as the polarizing field B.sub.0, but which have a gradient along the respective x, y and z axes. By controlling the strength of these gradients during each NMR cycle, the spatial distribution of spin excitation can be controlled and the location of the NMR signals can be identified.
NMR has rapidly developed into an imaging modality which is utilized to obtain tomographic, projection and volumetric images of anatomical features of live human subjects. Such images depict the nuclear-spin distribution (typically protons associated with water and fat), modified by specific NMR properties of tissues, such as spin-lattice (T.sub.1), and spin-spin (T.sub.2) relaxation time constants. They are of medical diagnostic value because they depict anatomy and allow tissue characterization.
The NMR scanners which implement these NMR techniques are constructed in a variety of sizes. Small, specially designed machines, are employed to examine laboratory animals or to provide images of specific parts of the human body. On the other hand, "whole body" NMR scanners are sufficiently large to receive an entire human body and produce an image of any portion thereof.
There are a number of techniques employed to produce the RF excitation field (B.sub.1) and receive the NMR signal. The simplest and most commonly used structure is a single coil and associated tuning capacitor which serves to both produce the excitation signal and receive the resulting NMR signal. This resonant circuit is electronically switched between the excitation circuitry and the receiver circuitry during each measurement cycle. Such structures are quite commonly employed in both small NMR scanners and whole body NMR scanners.
It is also quite common to employ separate excitation coils and receiver coils. While such NMR scanners require additional hardware, the complexities of electronic switching associated with the use of a single coil are eliminated and specially designed coils may be employed for the excitation and receiver functions. For example, in whole body NMR scanners it is desirable to produce a circularly polarized excitation field (B.sub.1) by using coils which are orthogonally oriented, and which are driven with separate excitation signals that are phase shifted 90.degree. with respect to each other. Such an excitation field is not possible with a single coil.
It is very difficult to construct a large coil which has both a uniform and high sensitivity to the NMR signal produced in a whole body NMR scanner. As a result, another commonly used technique is to employ "surface" coils to either generate the RF excitation signal (B.sub.1), receive the resulting NMR signal, or both generate and receive. Such surface coils are relatively small and are constructed to produce the desired field or receive the NMR signal from a localized portion of the patient. For example, different surface coils may be employed for imaging the head and neck, legs and arms, or various internal organs. When used as a transmitter, the surface coil should produce a uniform, homogeneous RF excitation field throughout the region of interest, and when employed as a receiver, the surface coil should provide a relatively uniform sensitivity to the NMR signals produced by the spin throughout the region of interest.
A surface coil construction which satisfies these requirements is described in U.S. Pat. No. 4,694,255, issued on Sept. 15, 1987 and entitled "Radio Frequency Field Coil For NMR." This surface coil is characterized by a pair of spaced loop elements which are connected together by a series of conductive segments containing reactive components. The desired resonant frequency of this "cylindrical cage" surface coil is determined by the geometry of the loop elements and conductive segments, and the size of the reactive components. The bird cage surface coil is thus constructed to operate at a single RF frequency which corresponds to the Larmor frequency of the particular spin being studied or imaged.
Although the nuclei of hydrogen atoms in water and fat produce the strongest NMR signals, other nuclei such as .sup.19 F, .sup.13 C and .sup.31 P also produce useful signals at frequencies quite different than that of hydrogen. Analysis of the NMR signals produced by phosphorous nuclei can be particularly revealing since phosphorous is involved in many metabloic processes and can be used to monitor intra-cellular pH. Because the NMR signal from hydrogen nuclei is much stronger, it is the predominent spin resonance used to produce images or to localize the region in a patient from which NMR signals are being produced. Ideally, hydrogen pulse sequences are interleaved with spectroscopic pulse sequences for other nuclei such as .sup.13 P and the hydrogen NMR signals are employed to locate the region from which the spectroscopic NMR signals are emanating. Such interleaved pulse sequences require the production of RF excitation pulses at two different frequencies and the reception of NMR signals at two different frequencies.